The read and write head portions of the slider for use in a typical prior art magnetic disk recording system are built-up in layers using thin film processing techniques. In the typical process of fabricating thin film magnetic heads, a large number of heads are formed simultaneously on a wafer. After the basic structures are formed the wafer is cut into rows or individual heads called sliders.
The various photolithographic and thin film deposition and plating steps involved in the fabrication of a magnetic head require monitoring by precisely measuring thin film features formed on the wafer. Image-based metrology is typically performed using electron beam or ion beam systems. To maintain the precision and accuracy of measurements on these devices or systems, the image pixel dimension of a system is currently determined by measuring a pitch of a known or assigned dimension in the images of calibration wafers, but pitch structure is not reliable for pixel calibration due to ambiguous edge definition in the image. The ambiguity arises from a gradual contrast change. In addition a pitch standard does not provide calibration for angles.
Traditionally, a set of lines separated by a distance (pitch) is made on calibration wafers and used as reference features for calibration on electron beam and ion beam systems such as SEM, CD-SEM, failure review SEM and FIB. The lines are topographical in that they are raised above the surrounding surface. The lines are designed for fast electron/ion-beam scanning. It is sufficient for edge detection for dimensional measurement which does not require any information from a high quality image. However, the signal-to-noise ratio is low from quick electron/ion raster scans and the signals cannot be used to provide a high quality image for further dimensional and angular measurements required in data storage industry in which a high quality slow scan image is acquired with known image pixel dimension in the beginning of the measurement process. To use the current topographical pitch structure in slow scan imaging acquisition for image pixel dimension calibration, factors such as line top edge rounding, line sidewall angle, and line bottom foot all result in apparent broadening of the intensity transition at the edges. This inevitably results in ambiguity in image pixel dimension calibration.
In U.S. Pat. No. 5,043,586 to Giuffre, et al., electron beam lithography grids are described having grid lines coplanar with the surface of the grid body and laterally supported by grooves formed in the grid body. An oxide layer is etched through to or somewhat beyond the substrate surface by Reactive Ion Etching (RIE) forming an intaglio or engraved pattern in the substrate. The remainder of the resist is removed and further RIE etching is done using the oxide mask to fully form grooves. A dense metal such as gold or tungsten is layered over the surface of the substrate including the grooves. The choice of material is based on the contrast of electron backscattering relative to the substrate material. The described prior art includes a calibration grid formed by an array of orthogonal, raised lines of gold on a substrate or body which is typically of silicon or similar semiconductor material. Measurement of beam position is accomplished by detecting changes in backscattering of electrons as the beam is swept across the calibration grid. The process of fabricating the calibration grids typically includes the formation of a multilayer resist including a stand-off layer, placed on the grid surface of a substrate, an intervening layer placed on the stand-off layer and an imaging layer placed on the intervening layer. To improve adhesion of gold to areas of the substrate or body, it is common to also include a thin layer of chromium between the surface of the substrate, either before applying the resist or at least before deposition of the gold. The grid is highly conductive to avoid electrical charges which can cause local deflection of the electron beam, causing significant errors in calibration.
U.S. Pat. No. 6,420,703 to Wu, et al. describes a method for forming a critical-dimension scanning electron microscope (CD-SEM) calibration standard with a plurality of metal lines formed of a suitable metal such as W, Pt, Au, Ta or Ti. The calibration standard is formed by a focused ion beam technique to produce straight, narrow lines. The substrate that has a planar top surface and the metal lines are formed on the planar top surface.
In U.S. Pat. No. 6,420,702 to Tripsas, et al., an SEM measurement standard utilizes two different conducting materials in order to prevent charging effects. The top material is selected to use grain morphology to focus secondary electrons, and to obtain improved image contrast. The lines are raised above the surrounding surface.
U.S. Pat. No. 5,528,047 to Nakajima describes an electron beam exposure apparatus with a stage including a reference marker composed of a base section and a projecting section. The base section is formed of a thin film of first conductive element having an atomic number (Z) greater than that of a material of the stage and has a first thickness through which more than 70% electrons in the beam can transmit. The projecting section is raised above the surrounding surface and is formed of a bulk of second conductive element having an atomic number equal to or greater than that of the material of the stage. The projecting section is made of a heavy material such as tungsten (W) having an atomic number greater than that of the material of stage. The heavy material effectively reflects electrons of an incident electron beam. The projecting section needs to be conductive. Tantalum (Ta), molybdenum (Mo), and chrome (Cr) or their alloys are also suggested as alternatives to tungsten.
High atomic number metal elements are described as electron beam registration alignment marks on low atomic number substrates in U.S. Pat. No. 4,123,661 to Wolf, et al. The combination produces enhanced secondary and backscattered electron video signals over topographical alignment marks of homogeneous materials. To augment the enhanced signal contrast, pairs of alignment marks are placed very close together to enhance the signal contrast. High atomic number materials, such as tantalum, tungsten, rhenium, osmium, iridium, platinum, gold and thallium are suggested for use on substrate materials of low atomic numbers, such as silicon, gallium arsenide and germanium. The alignment marks are shown embedded in photoresist.
Calibration standards are commercially available from Electron Microscopy Sciences in Hatfield, Pa. One of their products is an MXS 301BE which has alternating lines of two different elements which is said to provide excellent image contrast, and the titanium layer thickness is kept to 20 nm to control edge distortion effects in the SEM image. These physical characteristics make the edges sharp and readily discernable. The pattern is a direct recording of a laser-generated interference pattern which has been transferred into the 20 nm thick titanium film.